This invention relates to a nuclear reactor constructured to be capable of rapidly detecting an abnormalty of a core, particularly, an abnormality in the location of a fuel assembly.
A conventional nuclear reactor, for example a fast breeder, comprises a reactor 1, a core 3 formed by erecting a fuel assembly in the reactor vessel 1, an upper mechanism 4 mounted about the core 3 in a manner to face the core 3, and a rotatable plug 2 closing the upper opening of the reactor vessel 1, as shown in FIG. 1. A coolant 9 consisting of liquid sodium enters the reactor vessel through a lower inlet port (a) and flows out of the reactor vessel through an upper outlet port (b). As shown in the drawing, an ultrasonic transducer 7 and a reflective member 6, which collectively serve to detect the location and condition of a fuel assembly 5, are provided inside the reactor vessel 1 in a manner to face each other. It is seen that the transducer 7 and reflective member 6 are located to be suitable for inspection of the region between the upper mechanism 4 and the core 3. The ultrasonic transducer 7 is connected to a driving member 8 through a cable (c).
As mentioned above, the ultrasonic transducer 7 and the reflective member 6 collectively serve to detect the conditions of the fuel assembly 5. Specifically, the upper mechanism 4 is kept joined to the core 3 via a control bar or plate during operation of the nuclear reactor. Thus, for replacing the fuel assembly, it is necessary to move the upper mechanism 4 after it is detached from the core 3. It is very important to confirm the mechanical detachment of the upper mechanism 4 from the core 3 before moving the upper mechanism 4, because the fuel assembly mounted to the core 3 is sometimes caused to float so as to reach the upper mechanism 4 by thermal deformation or by the coolant 9 during operation of the nuclear reactor. Naturally, it is very dangerous to move the upper mechanism 4 when it is not sufficiently detached from the core 3. The ultrasonic transducer 7 and the reflective member 6 are used for confirming whether the upper mechanism 4 has been completely detached from the core 3. This method utilizes the ultrasonic wave generated from and received by the ultrasonic transducer 7 and comprises two types of operation.
In one type of operation, the ultrasonic transducer 7 alone is used for detecting the presence of obstacles such as the fuel assembly and control bar between the upper mechanism 4 and the core 3. Namely, the ultrasonic wave generated from the ultrasonic transducer 7 is reflected by such as obstacle, if present, and the reflective wave, or the echo, is received by the ultrasonic transducer 7. In this case, however, the detection accuracy is markedly influenced by the shape of the obstacle. For example, the fuel assembly 5 is a column hexagonal in cross section. Thus, the ultrasonic wave hitting the fuel assembly 5 is reflected in many directions, resulting in that the echo is very unlikely to return to the ultrasonic transducer 7. Further, it possibly happens that the floating fuel assembly is inclined with respect to the propagation direction of the ultrasonic wave as shown in FIG. 1. In this case, the echo travels in the direction shown by "X", resulting in failure to detect the presence of the inclined fuel assembly.
The other type of operation utilizes the reflective member 6 together with the ultrasonic transducer 7. Namely, the ultrasonic wave generated from the ultrasonic transducer 7 is reflected by the reflective member 6 to return to the transducer 7. If there is an obstacle in the passageway of the ultrasonic wave, the echo from the reflective member 6 fails to reach the transducer 7, thereby detecting the presence of the obstacle. Unlike the operation utilizing the transducer 7 alone, this type of operation relies on the absence of the echo received by the ultrasonic transducer. It follows that the presence of an obstacle can be detected regardless of the shape or inclination of the obstacle.
However, the operation involving the reflective member 6 necessitates an extremely high accuracy in mounting the reflective member. It is important to note that the reflective member should reflect the ultrasonic wave to return to the transducer. To achieve the object, it is customary to use a reflective member having the reflective surface curved to form an arc of a circle. Namely, distances of any optional points of the reflective surface from the transducer should be the same. Further, the reflective member must be mounted very accurately to permit the reflective wave to return to the ultrasonic transducer. What should also be considered is thermal deformation of the reflective member. If the reflective member fails to be positioned accurately either by inaccuracy in the mounting step or by thermal deformation, the echo fails to return to the transducer. Naturally, it is impossible to detect the presence of an obstacle in this case. Under the circumstances, it is required that the reflective member be mounted very accurately and, in addition, be adjusted very accurately when thermally deformed. It is naturally desired to alleviate the severity of the demands.